Superabsorbent polymer composition and method for preparing the same

ABSTRACT

The present invention relates to superabsorbent polymer and a method for preparing the same. The present invention can provide a superabsorbent polymer in which a hydrophobic material having an HLB of 0-6 and a surface cross-linking agent are mixed into a base resin prepared in the presence of water dispersible silica, thereby having improved rewetting characteristics and permeability through surface-modification of the base resin.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/KR2018/015468 filed Dec. 7, 2018which claims priority to Korean Patent Application No. 10-2017-0169490filed on Dec. 11, 2017 and Korean Patent Application No. 10-2018-0155292filed on Dec. 5, 2018 with the Korean Intellectual Property Office, thedisclosures of which are herein incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to superabsorbent polymer and a method forpreparing the same. More specifically, the present invention relates tosuperabsorbent polymer having improved rewet property and vortex time,and a method for preparing the same.

(b) Description of the Related Art

Super absorbent polymer (SAP) is synthetic polymer material that canabsorb moisture of 500 to 1000 times of self-weight, and is also nameddifferently as super absorbency material (SAM), absorbent gel material(AGM), etc. according to developing companies. The superabsorbentpolymer began to be commercialized as sanitary items, and currently, itis being widely used as hygienic goods such as a disposable diaper andthe like, water-holding material for soil, water stop material for civilengineering and architecture, sheets for raising seedling, freshnesspreservatives in the field of food circulation, fomentation material,and the like.

In most cases, such superabsorbent polymer is being widely used in thefield of hygienic goods such as a diaper or sanitary pad, etc., and forsuch use, it is required to exhibit high absorption power to moisture,and the like, and the absorbed moisture should not escape even underexternal pressure, and besides, it should properly maintain the shapeeven when it absorbs water and the volume is expanded (swollen), thusexhibiting excellent permeability.

However, it is known that centrifuge retention capacity (CRC) indicatingthe basic absorption power and water retention power of superabsorbentpolymer, and absorption under load (AUL) indicating the property ofretaining absorbed moisture despite the external pressure are difficultto be simultaneously improved. In case the whole crosslinking density ofsuperabsorbent polymer is controlled low, centrifuge retention capacitymay become relatively high, but the crosslink structure may becomeloose, and gel strength may decrease, thus deteriorating absorptionunder pressure. To the contrary, in case the crosslinking density iscontrolled high to improve absorption under pressure, it may becomedifficult to absorb moisture between the dense crosslink structures,thus deteriorating centrifuge retention capacity. For these reasons,there is a limit in providing superabsorbent polymer havingsimultaneously improved centrifuge retention capacity and absorptionunder pressure.

However, with the recent thinning of hygienic goods such as diapers andsanitary pads, superabsorbent polymer is required to have higherabsorption performances. Among them, it is an important problem tosimultaneously improve the conflicting properties of centrifugeretention capacity and absorption under pressure, and improvepermeability, and the like.

And, to the hygienic goods such as diapers or sanitary pads, and thelike, pressure may be applied by the weight of a user. Particularly, ifsuperabsorbent polymer applied for a diaper or a sanitary pad absorbsliquid, and then, a pressure is applied by the weight of a user, rewetphenomenon wherein a part of the liquid absorbed in the superabsorbentpolymer exudes again, and urine leakage may be generated.

Thus, many attempts are being made to inhibit such a rewet phenomenon.However, a specific method of effectively inhibiting rewet phenomenonhas not been suggested yet.

SUMMARY OF THE INVENTION

In order to solve the problem of the prior art, it is an object of thepresent invention to provide superabsorbent polymer in which rewet andurine leakage are inhibited, and a method for preparing the same.

In order to achieve the objects, one aspect of the present inventionprovides a method for preparing superabsorbent polymer comprising thesteps of:

preparing base resin in which acrylic acid-based monomers having acidgroups, of which at least a part are neutralized, and an internalcrosslinking agent are crosslinked, in the presence of colloidal silica(step 1);

mixing hydrophobic material having HLB of 0 or more and 6 or less, and asurface crosslinking agent with the base resin(step 2); and

increasing the temperature of the mixture of step 2 to conduct surfacemodification of the base resin (step 3).

And, another aspect of the present invention provides superabsorbentpolymer comprising:

base resin comprising crosslinked polymer in which acrylic acid-basedmonomers having acid groups, of which at least a part are neutralized,are crosslinked; and

a surface modification layer formed on the surface of the base resinparticles, in which the crosslinked polymer is additionally crosslinkedby a surface crosslinking agent,

wherein the surface modification layer comprises hydrophobic materialhaving HLB of 0 or more and 6 or less, and

a vortex time is 35 seconds or less.

According to the superabsorbent polymer and the preparation method ofthe present invention, superabsorbent polymer exhibiting excellentabsorption properties, in which rewet and urine leakage are inhibited,can be provided.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Although various modifications can be made to the present invention andthe present invention may have various forms, specific examples will beillustrated and explained in detail below. However, it should beunderstood that these are not intended to limit the present invention tospecific disclosure, and that the present invention includes all themodifications, equivalents or replacements thereof without departingfrom the spirit and technical scope of the invention.

Hereinafter, a method for preparing superabsorbent polymer compositionaccording to specific embodiments of the present invention will beexplained in more detail.

The method for preparing superabsorbent polymer according to oneembodiment of the present invention comprises the steps of:

preparing base resin in which acrylic acid-based monomers having acidgroups, of which at least a part are neutralized, and an internalcrosslinking agent are crosslinked, in the presence of colloidal silica(step 1);

mixing hydrophobic material having HLB of 0 or more and 6 or less, and asurface crosslinking agent with the base resin(step 2); and

increasing the temperature of the mixture of step 2 to conduct surfacemodification of the base resin (step 3).

Throughout the specification, “base resin” or “base resin powder” meanspolymer of water soluble ethylenically unsaturated monomers, made in theform of particles or powder by drying and grinding, and which is notsubjected to a surface modification or surface crosslinking stepdescribed below.

The hydrogel polymer obtained by the polymerization reaction of acrylicacid-based monomers may be subjected to the processes of drying,grinding, sieving, surface crosslinking, and the like, andcommercialized as a powder superabsorbent polymer product.

Recently, how long surface dryness can be maintained while diapers arepractically used, as well as absorption properties of superabsorbentpolymer such as centrifuge retention capacity, permeability, and thelike, is becoming an important measure for estimating the properties ofdiapers.

It was confirmed that the superabsorbent polymer obtained by thepreparation method of one embodiment has excellent centrifuge retentioncapacity, absorption under pressure, permeability, and the like, thusexhibiting excellent absorption performance, maintains dryness evenafter swollen by brine, and can effectively prevent rewet and urineleakage.

In the preparation method of superabsorbent polymer of the presentinvention, the raw material of the superabsorbent polymer, namely, themonomer composition comprising acrylic acid-based monomers having acidgroups, of which at least a part are neutralized, an internalcrosslinking agent, a polymerization initiator, and colloidal silica ispolymerized to obtain hydrogel polymer, and dried, ground and sieved toprepare base resin (step 1).

Hereinafter, it will be explained in more detail.

The monomer composition, which is the raw material of the superabsorbentpolymer, comprises acrylic acid-based monomers having acid groups, ofwhich at least a part are neutralized, and a polymerization initiator.

The acrylic acid-based monomer is a compound represented by thefollowing Chemical Formula 1:R¹—COOM¹  [Chemical Formula 1]

In the Chemical Formula 1,

R¹ is a C2-5 alkyl group comprising an unsaturated bond,

M¹ is a hydrogen atom, a monovalent or divalent metal, an ammonium groupor an organic amine salt.

Preferably, the acrylic acid-based monomers may be one or more selectedfrom the group consisting of acrylic acid, methacrylic acid, andmonovalent metal salts, divalent metal salt, ammonium salts and organicamine salts of these acids.

Here, the acrylic acid-based monomers may have acid groups, and at leasta part of the acid groups may be neutralized. Preferably, monomers thatare partially neutralized with alkali substance such as sodiumhydroxide, potassium hydroxide, ammonium hydroxide, etc. may be used.Here, the neutralization degree of the acrylic acid-based monomers maybe 40 to 95 mol %, or 40 to 80 mol %, or 45 to 75 mol %. Although therange of the neutralization degree may vary according to the finalproperties, if the neutralization degree is too high, neutralizedmonomers may be precipitated, thus rendering smooth progression ofpolymerization difficult, and to the contrary, if the neutralizationdegree is too low, the absorption power of the polymer may besignificantly lowered, and the polymer may exhibit rubber-like property,which is difficult to handle.

The concentration of the acrylic acid-based monomers may be about 20 toabout 60 wt %, preferably about 40 to about 50 wt %, based on themonomer composition comprising the raw materials of superabsorbentpolymer and solvents, and it may be controlled to an appropriateconcentration considering a polymerization time and reaction conditions,and the like. However, if the concentration of the monomers becomes toolow, the yield of superabsorbent polymer may decrease and economicefficiency may be lowered, and if it becomes too high, process problemsmay be generated such as precipitation of a part of the monomers or lowgrinding efficiency during grinding of the polymerized hydrogel polymer,and the properties of superabsorbent polymer may be deteriorated.

A polymerization initiator that is used in the preparation method ofsuperabsorbent polymer is not specifically limited as long as it iscommonly used for the preparation of superabsorbent polymer.

Specifically, as the polymerization initiator, a thermal polymerizationinitiator or a photopolymerization initiator by UV irradiation may beused according to a polymerization method. However, even in the case ofphotopolymerization, since a certain amount of heat is generated by UVirradiation, etc., and heat is generated to some degree according to theprogression of an exothermic polymerization reaction, a thermalpolymerization initiator may be additionally included.

The photopolymerization initiator is not limited in terms of itsconstruction, as long as it is a compound capable of forming a radicalby light such as UV.

As the photopolymerization initiator, one or more selected from thegroup consisting of benzoin ether, dialkyl acetophenone, hydroxylalkylketone, phenyl glyoxylate, benzyl dimethyl Ketal, acyl phosphine,and α-aminoketone may be used. Among them, as the acyl phosphine,commercially available lucirin TPO, i.e.,2,4,6-trimethyl-benzoyl-trimethyl phosphine oxide may be used. Morevarious photopolymerization initiators are described in ReinholdSchwalm's book, “UV Coatings: Basics, Recent Developments and NewApplication (Elsevier 2007)”, page 115, and are not limited to the abovedescribed examples.

The photopolymerization initiator may be included in the concentrationof about 0.01 to about 1.0 wt %, based on the monomer composition. Ifthe concentration of the photopolymerization initiator is too low,polymerization speed may become slow, and if it is too high, themolecular weight of superabsorbent polymer may be small and theproperties may become irregular.

And, as the thermal polymerization initiator, at least one selected fromthe group consisting of a persulfate initiator, an azo initiator,hydrogen peroxide, and ascorbic acid may be used. Specific examples ofthe persulfate initiator may include sodium persulfate (Na₂S₂O₈),potassium persulfate (K₂S₂O₈), ammonium persulfate ((NH₄)₂S₂O₈), etc.,and, specific examples of the azo initiator may include2,2-azobis(2-amidinopropane)dihydrochloride,2,2-azobis-(N,N-dimethylene)isobutyramidinedihydrochloride,2-(carbamoylazo)isobutyronitril,2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride,4,4-azobis-(4-cyanovalericacid), etc. More various thermal initiatorsare described in “Principle of Polymerization (Wiley, 1981)”, Odian'sbook, page 203, and are not limited to the above described examples.

According to one embodiment of the invention, the monomer compositionmay further comprise an internal crosslinking agent as the raw materialof superabsorbent polymer. As the internal crosslinking agent, acrosslinking agent having one or more functional groups capable ofreacting with the acrylic acid-based monomers, and having one or moreethylenically unsaturated groups; or a crosslinking agent having two ormore functional groups capable of reacting with the substituents of theacrylic acid-based monomers and/or the substituents formed by thehydrolysis of the monomers may be used.

Specific examples of the internal crosslinking agent may include one ormore selected from the group consisting of N,N′-methylenebisacrylamide,trimethylolpropane tri(meth)acrylate, ethyleneglycol di(meth)acrylate,polyethyleneglycol (meth)acrylate, propyleneglycol di(meth)acrylate,polypropyleneglycol (meth)acrylate, butanediol di(meth)acrylate,butyleneglycol di(meth)acrylate, diethyleneglycol di(meth)acrylate,hexanediol di(meth)acrylate, triethyleneglycol di(meth)acrylate,tripropyleneglycol di(meth)acrylate, tetraethyleneglycoldi(meth)acrylate, dipentaerythritol pentaacrylate, glycerintri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine,ethyleneglycol diglycidyl ether, propylene glycol, glycerin, andethylene carbonate.

Such an internal crosslinking agent may be included in the concentrationof about 0.01 to about 0.5 wt % based on the monomer composition, tocrosslink polymerized polymer.

In the preparation method of the present invention, the monomercomposition comprises colloidal silica.

The colloidal silica means silica wherein silica particles are stablydispersed in water without precipitation or aggregation, and at least apart of the surfaces of the silica particles are ionized. Thepreparation method of the colloidal silica is not specifically limited,and any known methods such as electrodialysis, a sol-gel process, an ionexchange process, an acid neutralization process, and the like may beused.

The particle diameter of the colloidal silica is preferably about 5 nmor more, or about 10 nm or more, and about 100 nm or less, or about 50nm or less, or about 30 nm or less. If the particle diameter of thecolloidal silica is too small, production cost may increase due toexpensive cost, and if it is too large, the effect of rewet improvementmay not be obtained.

And, the colloidal silica may be added in the concentration of about0.01 to about 1.0 part by weight, or about 0.02 to about 0.5 parts byweight, based on 100 parts by weight of the acrylic acid-based monomers.If the amount of the colloidal silica used is greater than 1.0 part byweight, the centrifuge retention capacity of superabsorbent polymer maybe deteriorate, and if it is less than 0.01 parts by weight, the rewetimprovement effect may not be obtained, and thus, the above range may bepreferable.

And, if the colloidal silica fails to maintain a colloidal state and isprecipitated in a monomer composition, the rewet improvement effect maynot be obtained, and thus, it is preferable to use colloidal silica thatmaintains a stable colloidal state in a monomer composition. Thus,powder or hydrophobic silica instead of colloidal silica does not haverewet improvement effect, and thus, cannot achieve the effect intendedin the present invention.

As the colloidal silica fulfilling such a requirement, ST-O, ST-AK, andthe like (Nissan Chemical Corporation) may be mentioned, but the presentinvention is not limited thereto.

According to the preparation method of the present invention, by addingthe above explained colloidal silica during polymerization andconducting polymerization in the presence of the colloidal silica, theeffect of improving the gel strength of superabsorbent polymer particlesby the hydrogen bonds between superabsorbent polymer and colloidalsilica may be obtained, and thus, rewet property may be improved.

In the preparation method of the present invention, the monomercomposition may further comprise a foaming agent, and/or a foamstabilizer.

The foaming agent performs a function for foaming during polymerizationto form pores in hydrogel polymer, thus increasing the surface area. Asthe foaming agent, carbonate may be used, and for example, sodiumbicarbonate, sodium carbonate, potassium bicarbonate, potassiumcarbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonateor magnesium carbonate may be used.

And, the foaming agent may be added in the concentration of about 0.005to about 1 part by weight, or about 0.01 to about 0.3 parts by weight,based on 100 parts by weight of the acrylic acid-based monomers. If theamount of the foaming agent used is greater than 1 part by weight, theremay be too many pores, and thus, the gel strength and density ofsuperabsorbent polymer may decrease, thus causing problems in terms ofdistribution and storage. And, if it is less than 0.005 parts by weight,the function as a foaming agent may be insignificant.

And, the foam stabilizer performs functions for maintaining the shape ofthe foams formed by the foaming agent, and simultaneously, uniformlydistributing the foams over the whole area of polymer, therebyincreasing the surface area of polymer.

As the foam stabilizer, anionic surfactant may be used, and as theexamples of anionic surfactant that can be used, sodium dodecyl sulfate,sodium stearate, ammonium lauryl sulfate, sodium lauryl ether sulfate,sodium myreth sulfate, or alkylether sulfate-based compounds similar tothe above compounds may be mentioned. The anionic surfactant that can beused is not limited thereto, but preferably, sodium dodecyl sulfate orsodium stearate may be used.

The anionic surfactant may be added in the concentration of about 0.001to about 1 part by weight, or about 0.005 to about 0.05 parts by weight,based on 100 parts by weight of the acrylic acid-based monomers. If theconcentration of the anionic surfactant is too low, the function as afoam stabilizer may be insignificant, and thus, it may be difficult toachieve the effect of improving absorption speed, and to the contrary,if the concentration is too high, surface tension after polymerizationmay excessively decrease, thus adversely influencing the properties of adiaper.

In the preparation method, the monomer composition may further compriseadditives such as a thickener, a plasticizer, a preservation stabilizer,an antioxidant, etc., as necessary.

The above explained raw materials such as acrylic acid-based monomershaving acid groups, of which at least part are neutralized, aphotopolymerization initiator, a thermal polymerization initiator, aninternal crosslinking agent, and additives may be prepared in the formof a solution dissolved in a solvent.

Here, the solvent that can be used is not limited in terms of itsconstruction as long as it can dissolve or disperse the above explainedraw materials, and for example, one or more selected from water,ethanol, ethyleneglycol, diethyleneglycol, triethyleneglycol,1,4-butanediol, propyleneglycol, ethyleneglycol monobutyl ether,propyleneglycol monomethyl ether, propyleneglycol monomethyl etheracetate, methylethylketone, acetone, methylamylketone, cyclohexanone,cyclopentanone, diethyleneglycol monomethyl ether, diethyleneglycolethyl ether, toluene, xylene, butyrolactone, carbitol, methylcellosolveacetate and N,N-dimethylacetamide, etc. may be used alone or incombination.

The solvent may be included in the remaining amount except the abovedescribed components, based on the total content of the monomercomposition.

Meanwhile, a method of forming hydrogel polymer by the thermalpolymerization or photopolymerization of the monomer composition is notspecifically limited in terms of its construction, as long as it is acommonly used polymerization method.

Specifically, the polymerization method is largely classified intothermal polymerization and photopolymerization according to an energysource. Commonly, thermal polymerization may be progressed in a reactorequipped with a stirring axis such as a kneader, and,photopolymerization may be progressed in a reactor equipped with amovable conveyer belt, but the above explained polymerization methodsare no more than examples, and the present invention is not limitedthereto.

For example, hydrogel polymer may be obtained by introducing the abovedescribed monomer composition into a reactor equipped with a stirringaxis such as a kneader, and supplying hot air or heating the reactor toprogress thermal polymerization. Here, the hydrogel polymer dischargedto the outlet of the reactor may in the size of a few centimeters to afew millimeters according to the shape of the stirring axis equipped inthe reactor. Specifically, the size of obtained hydrogel polymer mayvary according to the concentration of the introduced monomercomposition and the introduction speed, etc., and commonly, hydrogelpolymer with a (weight average) particle diameter of 2 to 50 mm may beobtained.

And, in case photopolymerization of the monomer composition isprogressed in a reactor equipped with a movable conveyer belt asexplained above, hydrogel polymer in the form of a sheet having a widthof the belt may be obtained. Here, the thickness of the sheet may varyaccording to the concentration of the introduced monomer composition andthe introduction speed, but it is preferable that the monomercomposition is fed so as to obtain polymer in the form of sheet having athickness of about 0.5 to about 5 cm. If a monomer composition is fed sothat the thickness of sheet polymer may become too thin, productionefficiency may be low, and if the thickness of sheet polymer is greaterthan 5 cm, due to the excessively thick thickness, polymerization maynot uniformly occur over the whole thickness.

Here, the moisture content of hydrogel polymer obtained by such a methodmay be about 40 to about 80 wt %. Throughout the specification, the“moisture content” is the content of moisture occupied based on thetotal weight of hydrogel polymer, and it means a value obtained bysubtracting the weight of polymer of a dry state from the weight ofhydrogel polymer. Specifically, it is defined as a value calculated bymeasuring the weight loss according to moisture evaporation in thepolymer while raising the temperature of polymer through infraredheating to dry. At this time, the drying condition is set up such thatthe temperature is raised from room temperature to about 180° C. andthen maintained at 180° C., and the total drying time is 20 minutesincluding a temperature raising step of 5 minutes.

Next, the obtained hydrogel polymer is dried.

Wherein, a coarse grinding step may be further conducted before dryingthe hydrogel polymer so as to increase drying efficiency.

Here, grinders that can be used in the coarse grinding is not limited interms of the constructions, but specifically, one selected from thegroup consisting of a vertical pulverizer, a turbo cutter, a turbogrinder, a rotary cutter mill, a cutter mill, a disc mill, a shredcrusher, a crusher, a chopper, a disc cutter may be used, but thegrinder is not limited thereto.

Through the coarse grinding step, the particle diameter of the hydrogelpolymer may be controlled to about 2 to about 10 mm.

Grinding to a particle diameter less than 2 mm would not be technicallyeasy due to the high moisture content of hydrogel polymer, and causeagglomeration between ground particles. Meanwhile, if grinding to aparticle diameter greater than 10 mm, the effect for increasing theefficiency in the subsequent drying step may be insignificant.

The hydrogel polymer coarsely ground as explained above, or hydrogelpolymer immediately after polymerization that is not subjected to thecoarse grinding step is dried. Here, the drying temperature may be about150 to about 250° C. If the drying temperature is less than 150° C., adrying time may become excessively long, and the properties of thefinally formed superabsorbent polymer may be deteriorated, and if thedrying temperature is greater than 250° C., only the surface of polymermay be dried to generate fine powders in the subsequent grindingprocess, and the properties of the finally formed superabsorbent polymermay be deteriorated. Thus, it is preferable that the drying isprogressed at a temperature of about 150 to about 200° C., morepreferably about 160 to about 180° C.

Meanwhile, a drying time may be about 20 to about 90 minutes,considering process efficiency, and the like, but is not limitedthereto.

And, the drying method is not limited in terms of the construction aslong as it is commonly used as a drying process of hydrogel polymer.Specifically, the drying step may be progressed by hot wind supply,infrared ray irradiation, ultrahigh frequency wave irradiation, or UVirradiation, etc. The polymer dried by such a method may exhibit amoisture content of about 0.1 to about 10 wt %.

Next, the dried polymer obtained through the drying step is ground.

The particle diameter of the polymer powder obtained after the grindingstep may be 150 μm to 850 μm. As a grinder for grinding to such aparticle diameter, specifically, a pin mill, a hammer mill, a screwmill, a roll mill, a disc mill, or a jog mill, etc. may be used, but thegrinder is not limited thereto.

And, in order to manage the properties of the superabsorbent polymerpowders finally productized after the grinding step, the polymer powdersobtained after grinding may be subjected to a separate process ofsieving according to the particle diameter.

Next, hydrophobic material having HLB of 0 or more and 6 or less, and asurface crosslinking agent are mixed with the base resin(step 2).

In a common preparation method of superabsorbent polymer, dried andground polymer, namely, base resin is mixed with a surface crosslinkingsolution comprising a surface crosslinking agent, and then, the mixtureis heated to raise the temperature, thereby conducting a surfacecrosslinking reaction of the ground polymer.

The surface crosslinking step is a step of inducing a crosslinkingreaction on the surface of ground polymer in the presence of a surfacecrosslinking agent, thereby forming superabsorbent polymer having moreimproved properties. Through the surface crosslinking, a surfacecrosslink layer (surface modification layer) is formed on the surface ofground polymer particles.

In general, since a surface crosslinking agent is coated on the surfaceof superabsorbent polymer particles, a surface crosslinking reactionoccurs on the surface of superabsorbent polymer particles, and itimproves crosslinkability on the surface of the particles withoutsubstantially influencing the inside of the particles. Thus, surfacecrosslinked superabsorbent polymer particles have higher crosslinkingdegree around the surface than inside.

However, although absorption under pressure and permeability may beimproved by the surface crosslinking reaction, rewet property andcentrifuge retention capacity may be deteriorated.

Meanwhile, according to the preparation method of the present invention,rewet property can be improved by mixing hydrophobic material with baseresin, before mixing a surface crosslinking agent with the base resin toconduct a surface crosslinking reaction. And, the surface crosslinkingefficiency may be improved, and thus, absorption speed and permeabilitycan be further improved, compared to the resin without using hydrophobicmaterial.

As the hydrophobic material, materials fulfilling the lower limit of HLBof 0 or more, or 1 or more, or 2 or more, and the upper limit of 6 orless, or 5 or less, or 5.5 or less may be used. And, since thehydrophobic material should be dissolved during the surface crosslinkingreaction and positioned in the surface modification layer of the baseresin, materials of which melting point is below the surfacecrosslinking reaction temperature may be used.

Examples of hydrophobic materials that can be used may include glycerylstearate, glycol stearate, magnesium stearate, glyceryl laurate,sorbitan stearate, sorbitan trioleate, and PEG-4 dilaurate, and thelike, and preferably, glyceryl stearate, or glyceryl laurate may beused, but the present invention is not limited thereto.

The hydrophobic material is distributed in the surface modificationlayer of the surface of the base resin, and it may prevent theaggregation or agglomeration of swollen polymer particles by increasedpressure, while the superabsorbent polymer absorbs liquid and isswollen, and may afford hydrophobicity to the surface, therebyfacilitating the penetration and diffusion of liquid. Thus, it maycontribute to improvement in the rewet property of superabsorbentpolymer. And, since the hydrophobic material is coated on the surface,it makes difficult for liquid absorbed inside to move outside theswollen polymer. Thus, it may contribute to improvement in the rewetproperty of superabsorbent polymer.

The hydrophobic material may be mixed in the amount of about 0.02 partsby weight or more, or about 0.025 parts by weight or more, or about 0.05parts by weight or more, and about 0.5 parts by weight or less, or about0.3 parts by weight or less, or about 0.1 parts by weight or less, basedon 100 parts by weight of the base resin. If the amount of thehydrophobic material is less than 0.02 parts by weight, it may not besufficient for improving rewet property, and if the hydrophobic materialis excessively included exceeding 0.5 parts by weight, base resin andhydrophobic material may be separated from each other, and thus, rewetproperty improvement effect may not be obtained or the hydrophobicmaterial may act as impurities. Thus, the above range is preferable.

A method of mixing the hydrophobic material is not specifically limitedas long as it can uniformly mix it with the base resin.

For example, the hydrophobic material may be dry mixed before a surfacecrosslinking solution comprising a crosslinking agent is mixed with thebase resin, or it may be dispersed in the surface crosslinking solutiontogether with a surface crosslinking agent and mixed with the baseresin. Alternatively, separately from the surface crosslinking solution,the hydrophobic material may be heated above the melting point and mixedin a solution state.

When adding the surface crosslinking agent, it may be added in the formof a surface crosslinking solution by additionally mixing watertogether. When water is added, the surface crosslinking agent may beuniformly dispersed in the polymer. Here, it is preferable that thecontent of water added is about 1 to about 10 parts by weight, based on100 parts by weight of the polymer, so as to induce uniform dispersionof the surface crosslinking agent, preventing the agglomeration ofpolymer powders, and optimizing the surface penetration depth of thesurface crosslinking agent.

And, the surface crosslinking agent is not limited in terms of itsconstruction as long as it can react with the functional group of thepolymer

Preferably, in order to improve the properties of producedsuperabsorbent polymer, as the surface crosslinking agent, one or moreselected from the group consisting of polyhydric alcohol compounds;epoxy compounds; polyamine compounds; haloepoxy compounds; thecondensation products of haloepoxy compounds; oxazoline compounds;mono-, di- or polyoxazolidinone compounds; cyclic urea compounds;multivalent metal salts; and alkylene carbonate compounds may be used.

Specifically, as the polyhydric alcohol compound, one or more selectedfrom the group consisting of mono-, di-, tri-, tetra- or polyethyleneglycol, monopropylene glycol, 1,3-propanediol, dipropylene glycol,2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol,polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol,1,5-pentanediol, 1,6-hexanediol, and 1,2-cyclohexandimethanol may beused.

And, as the epoxy compound, one or more selected from the groupconsisting of ethylene glycol diglycidyl ether and glycidol, and thelike may be used, and as the polyamine compound, one or more selectedfrom the group consisting of ethylenediamine, diethylene triamine,triethylenetetraamine, tetraethylenepentamine, pentaethylenehexamine,polyethyleneimine, and polyamide polyamine may be used.

And, as the haloepoxy compound, epichlorohydrin, epibromohydrin, andα-methylepichlorohydrin may be used. And, as the mono-, di- orpolyoxazolidinone compounds, 2-oxazolidinone, and the like may be used.

And, as the alkylene carbonate compound, ethylene carbonate, and thelike may be used. These compounds may be used alone or in combinations.Meanwhile, in order to increase the efficiency of the surfacecrosslinking process, one or more kinds of polyhydric alcohol compoundshaving a carbon number of 2 to 10 may be included in the surfacecrosslinking agent.

The content of the surface crosslinking agent added may be appropriatelyselected according to the kinds of the surface crosslinking agent orreaction conditions, but it may be used in the amount of about 0.001 toabout 5 parts by weight, preferably about 0.01 to about 3 parts byweight, more preferably about 0.05 to about 2 parts by weight, based on100 parts by weight of the base resin.

If the content of the surface crosslinking agent is too small, a surfacecrosslinking reaction may hardly occur, and if is exceeds 5 parts byweight, based on 100 parts by weight of the polymer, due to theprogression of excessive surface crosslinking reaction, absorption powerand properties may be deteriorated.

Meanwhile, in addition to the above explained surface crosslinkingagent, multivalent metal salts, for example, one or more selected fromthe group consisting of aluminum salts, more specifically, sulfates,potassium salts, ammonium salts, sodium salts and hydrochloride may befurther included.

By additionally using such a multivalent metal salt, the permeability ofthe superabsorbent polymer prepared by the method of one embodiment canbe further improved. Such a multivalent metal salt may be added to thesurface crosslinking solution together with the surface crosslinkingagent, and it may be used in the amount of about 0.01 to 4 parts byweight, based on 100 parts by weight of the base resin.

Next, the mixture of base resin, a surface crosslinking agent, andhydrophobic material is heated to increase the temperature, therebyconducting a surface modification step of the base resin (step 3).

The surface modification step may be conducted by heating at atemperature of about 80 to about 190° C., preferably about 100 to about180° C. for about 10 to about 90 minutes, preferably about 20 to about70 minutes. If the crosslinking reaction temperature is less than 80° C.or the reaction time is too short, a surface crosslinking reaction maynot properly occur, and thus, permeability may decrease, and if thetemperature is greater than 190° C. or the reaction time is too long,centrifuge retention capacity may be deteriorated.

A temperature rise means for the surface modification reaction is notspecifically limited. A heating medium may be supplied, or a heat sourcemay be directly supplied to heat. Here, the kinds of the heating mediumthat can be used may include temperature-increased fluid such as steam,hot air, hot oil, etc., but are not limited thereto, and the temperatureof the heating medium supplied may be appropriately selected consideringthe means of the heating medium, temperature rise speed and atemperature to be increased. Meanwhile, the heat source directlysupplied may include electric heating, gas heating, etc., but is notlimited thereto.

According to the preparation method of the present invention, the baseresin may have excellent gel strength due to the colloidal silica addedduring polymerization, and by the surface modification step, on thesurface of the base resin, a surface crosslink structure that is formedby the reaction of the surface crosslinking agent and the functionalgroups of the base resin may be formed, and a surface modification layerin which the above explained hydrophobic material is uniformlydistributed in the surface crosslink structure may be formed.

Thus, the superabsorbent polymer prepared by the preparation method ofthe present invention may have improved rewet property and initialabsorption speed without deteriorating the properties such as centrifugeretention capacity and absorption under pressure, and the like, due tothe properties of the base resin and the surface modification layerformed on the base resin.

Thus, according to another embodiment of the present invention,superabsorbent polymer is provided, which comprises:

base resin comprising crosslinked polymer in which acrylic acid-basedmonomers having acid groups, of which at least a part are neutralized,are crosslinked; and

a surface modification layer formed on the surface of the base resinparticles, in which the crosslinked polymer is additionally crosslinkedby a surface crosslinking agent,

wherein the surface modification layer comprises hydrophobic materialhaving HLB of 0 or more and 6 or less, and

a vortex time is 35 seconds or less.

Specific preparation method and properties of the superabsorbent polymerare as explained above.

The superabsorbent polymer may have centrifuge retention capacity (CRC)measured according to EDANA method WSP 241.3, in the range of about 30g/g or more, or about 31 g/g or more, or about 32 g/g or more, and about40 g/g or less, or about 38 g/g or less, or about 35 g/g or less.

And, the superabsorbent polymer may have absorption under pressure (AUP)of 0.7 psi, measured according to EDANA method WSP 242.3, in the rangeof about 20 g/g or more, or about 22 g/g or more, or about 23 g/g ormore, and about 35 g/g or less, or about 33 g/g or less, or about 32 g/gor less.

And, the superabsorbent polymer may have a vortex time of 35 seconds orless, or 32 seconds or less, or about 30 seconds or less, or about 28seconds or less. The vortex time is more excellent as it is smaller, andthus, the lower limit of the vortex time is theoretically 0 second, butfor example, it may be about 5 seconds or more, or about 10 seconds ormore, or about 12 seconds or more.

The vortex time means a time (unit: second) taken until liquid vortexdisappears by rapid absorption, when superabsorbent polymer is added toa saline solution and stirred, and it is considered that as the time isshorter, superabsorbent polymer has more rapid initial absorption speed.

And, the superabsorbent polymer may have permeability measured accordingto the following Equation 1, of 20 seconds or less, or 18 seconds orless, or 16 seconds or less. The permeability is more excellent as thevalue is smaller, and thus, the lower limit is theoretically 0 second,but for example, it may be about 5 seconds or more, or about 10 secondsor more, or about 12 seconds or more.Permeability (sec)=T1−B  [Equation 1]

in the Equation 1,

T1 is a time taken until the height of a liquid level decreases from 40ml to 20 ml, after putting 0.2±0.0005 g of a sieved (30#˜50#)superabsorbent polymer sample in a chromatography column and addingbrine to the volume of 50 ml, and then, leaving it for 30 minutes; and Bis a time taken until the height of a liquid level decreases from 40 mlto 20 ml in a chromatography column filled with brine.

And, the superabsorbent polymer may exhibit excellent absorptionproperties, and simultaneously, exhibit more improved rewet property.

More specifically, the rewet property (tap water long-term rewet underno pressure) may be 3.0 g or less, or 2.5 or less, or 2.0 or less, saidrewet property being defined by the weight of water exuding fromsuperabsorbent polymer to a filter paper, after 1 g of thesuperabsorbent polymer is soaked in 100 g of tap water to swell for 10minutes, and then, the swollen superabsorbent polymer is left on thefilter paper for 3 hour from the first time when it is soaked in the tapwater. As the weight of water is smaller, the rewet property is moreexcellent, and thus, the lower limit is theoretically 0 g, but forexample, it may be 0.1 g or more, or 0.5 g or more, or 1.0 g or more.

And, the rewet property (tap water short-term rewet under no pressure)may be 1.0 g or less, or 0.9 g or less, or 0.8 g or less, said rewetproperty being defined by the weight of water exuding fromsuperabsorbent polymer to a filter paper, after 1 g of thesuperabsorbent polymer is soaked in 100 g of tap water to swell for 10minutes, and then, the swollen superabsorbent polymer is left on thefilter paper for 1 hour from the first time when it is soaked in the tapwater. As the weight of water is smaller, the rewet property is moreexcellent, and thus, the lower limit is theoretically 0 g, but forexample, it may be 0.1 g or more, or 0.2 g or more, or 0.3 g or more.

In the rewet property evaluation, the tap water used may have electricconductivity of 170 to 180 μS/cm. Since the electric conductivity of tapwater significantly influences the properties measured, there is a needto measure the properties such as rewet property using tap water havingelectric conductivity of an equivalent level.

As explained above, the superabsorbent polymer of the present inventionhas excellent absorption power, and even if it absorbs a large quantityof urine, rewet and urine leakage may be inhibited.

The present invention will be explained in more detail in the followingexamples. However, these examples are presented only as theillustrations of the present invention, and the scope of the presentinvention is not limited thereby.

EXAMPLE Preparation of Superabsorbent Polymer Example 1

100 g of acrylic acid, 0.6 g of polyethyleneglycol diacrylate (PEGDA,Mw=523) as a crosslinking agent, 0.008 g ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide as a photoinitiator,0.15 g of sodium persulfate (SPS) as a thermal initiator, 0.08 g ofsodium bicarbonate (SBC) as a foaming agent, 0.035 g of sodiumdodecylsulfate (SDS) as surfactant, 0.4 g of colloidal silica (ST-O),123.3 g of 31.5% caustic soda (NaOH), and 38.53 g of water were mixed toprepare an aqueous monomer composition. The aqueous monomer compositionwas received in a tray, and then, while maintaining a polymerizationatmosphere temperature of 80° C., it was irradiated by UV (irradiationamount: 10 mW/cm²) with a UV irradiation equipment for 1 minute, andaged for 2 minutes to progress UV polymerization, thus preparing ahydrogel polymer sheet.

The polymerized sheet was taken out and cut to a size of 3 cm×3 cm, andthen, chopped using a meat chopper to prepare crumb. The crumb was driedin an oven capable of transferring air volume up and down. Hot air of185° C. was flowed from the lower part to the upper part for 15 minutes,and flowed from the upper part to the lower part for 15 minutes, so thatthe crumb was uniformly dried, and the moisture content of the driedproduct became 2% or less. After drying, it was ground with a grinder,and then, sieved for 10 minutes with an amplitude of 1.5 mm (combinationof sieving mesh: #20-30/#30-50/#50-100), and each sieved part(10%/75%/15%) was collected to obtain polymer having a particle diameterof about 150 μm to 850 μm, thus obtaining base resin powder.

Thereafter, 0.02 parts by weight of glyceryl stearate, and a surfacecrosslinking solution (7.6 parts by weight of water, 7.6 parts by weightof methanol, 0.075 parts by weight of ethyleneglycol diglycidyl ether(EX-810), 0.03 parts by weight of sodium metabisulfite, 0.1 parts byweight of aluminum sulfate 18 hydrate (Al—S), and 0.03 parts by weightof aluminum oxide (Alu 130)) were uniformly mixed with 100 parts byweight of the base resin prepared above, and then, a surfacecrosslinking reaction was progressed at 140° C. for 35 minutes. Afterthe surface treatment was finished, superabsorbent polymer having anaverage particle diameter of 150 to 850 μm was obtained using a sieve.

Thereafter, 0.05 parts by weight of aluminum oxide (Alu 130) was drymixed with the superabsorbent polymer prepared above.

Example 2

100 g of acrylic acid, 0.6 g of polyethyleneglycol diacrylate (PEGDA,Mw=523) as a crosslinking agent, 0.008 g ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide as a photoinitiator,0.15 g of sodium persulfate (SPS) as a thermal initiator, 0.08 g ofsodium bicarbonate (SBC) as a foaming agent, 0.035 g of sodiumdodecylsulfate (SDS) as surfactant, 0.4 g of colloidal silica (ST-AK),123.3 g of 31.5% caustic soda (NaOH), and 38.53 g of water were mixed toprepare an aqueous monomer composition. The aqueous monomer compositionwas received in a tray, and then, while maintaining a polymerizationatmosphere temperature of 80° C., it was irradiated by UV (irradiationamount: 10 mW/cm²) with a UV irradiation equipment for 1 minute, andaged for 2 minutes to progress UV polymerization, thus preparing ahydrogel polymer sheet.

The polymerized sheet was taken out and cut to a size of 3 cm×3 cm, andthen, chopped using a meat chopper to prepare crumb. The crumb was driedin an oven capable of transferring air volume up and down. Hot air of185° C. was flowed from the lower part to the upper part for 15 minutes,and flowed from the upper part to the lower part for 15 minutes, so thatthe crumb was uniformly dried, and the moisture content of the driedproduct became 2% or less. After drying, it was ground with a grinder,and then, sieved for 10 minutes with an amplitude of 1.5 mm (combinationof sieving mesh: #20-30/#30-50/#50-100), and each sieved part(10%/75%/15%) was collected to obtain polymer having a particle diameterof about 150 μm to 850 μm, thus obtaining base resin powder.

Thereafter, 0.02 parts by weight of glyceryl stearate, and a surfacecrosslinking solution (7.6 parts by weight of water, 7.6 parts by weightof methanol, 0.075 parts by weight of ethyleneglycol diglycidyl ether(EX-810), 0.03 parts by weight of sodium metabisulfite, 0.1 parts byweight of aluminum sulfate 18 hydrate (Al—S), and 0.03 parts by weightof aluminum oxide (Alu 130)) were uniformly mixed with 100 parts byweight of the base resin prepared above, and then, a surfacecrosslinking reaction was progressed at 140° C. for 35 minutes. Afterthe surface treatment was finished, superabsorbent polymer having anaverage particle diameter of 150 to 850 μm was obtained using a sieve.

Thereafter, 0.05 parts by weight of aluminum oxide (Alu 130) was drymixed with the superabsorbent polymer prepared above.

Example 3

100 g of acrylic acid, 0.75 g of polyethyleneglycol diacrylate (PEGDA,Mw=523) as a crosslinking agent, 0.008 g ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide as a photoinitiator,0.08 g of sodium persulfate (SPS) as a thermal initiator, 0.2 g ofsodium bicarbonate (SBC) as a foaming agent, 0.025 g of sodiumdodecylsulfate (SDS) as surfactant, 0.1 g of colloidal silica (ST-O),123.3 g of 31.5% caustic soda (NaOH), and 38.53 g of water were mixed toprepare an aqueous monomer composition. The aqueous monomer compositionwas received in a tray, and then, while maintaining a polymerizationatmosphere temperature of 80° C., it was irradiated by UV (irradiationamount: 10 mW/cm²) with a UV irradiation equipment for 1 minute, andaged for 2 minutes to progress UV polymerization, thus preparing ahydrogel polymer sheet.

The polymerized sheet was taken out and cut to a size of 3 cm×3 cm, andthen, chopped using a meat chopper to prepare crumb. The crumb was driedin an oven capable of transferring air volume up and down. Hot air of185° C. was flowed from the lower part to the upper part for 15 minutes,and flowed from the upper part to the lower part for 15 minutes, so thatthe crumb was uniformly dried, and the moisture content of the driedproduct became 2% or less. After drying, it was ground with a grinder,and then, sieved for 10 minutes with an amplitude of 1.5 mm (combinationof sieving mesh: #20-30/#30-50/#50-100), and each sieved part(5%/75%/20%) was collected to obtain polymer having a particle diameterof about 150 μm to 850 μm, thus obtaining base resin powder.

Thereafter, 0.025 parts by weight of glyceryl stearate, and a surfacecrosslinking solution (7.7 parts by weight of water, 5.5 parts by weightof methanol, 0.02 parts by weight of ethyleneglycol diglycidyl ether(EX-810), 0.05 parts by weight of sodium metabisulfite, 0.1 parts byweight of aluminum sulfate 18 hydrate (Al—S), and 0.03 parts by weightof aluminum oxide (Alu 130)) were uniformly mixed with 100 parts byweight of the base resin prepared above, and then, a surfacecrosslinking reaction was progressed at 140° C. for 35 minutes. Afterthe surface treatment was finished, superabsorbent polymer having anaverage particle diameter of 150 to 850 μm was obtained using a sieve.

Thereafter, 0.05 parts by weight of aluminum oxide (Alu 130) was drymixed with the superabsorbent polymer prepared above.

Example 4

100 g of acrylic acid, 0.75 g of polyethyleneglycol diacrylate (PEGDA,Mw=523) as a crosslinking agent, 0.008 g ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide as a photoinitiator,0.08 g of sodium persulfate (SPS) as a thermal initiator, 0.2 g ofsodium bicarbonate (SBC) as a foaming agent, 0.025 g of sodiumdodecylsulfate (SDS) as surfactant, 0.1 g of colloidal silica (ST-AK),123.3 g of 31.5% caustic soda (NaOH), and 38.53 g of water were mixed toprepare an aqueous monomer composition. The aqueous monomer compositionwas received in a tray, and then, while maintaining a polymerizationatmosphere temperature of 80° C., it was irradiated by UV (irradiationamount: 10 mW/cm²) with a UV irradiation equipment for 1 minute, andaged for 2 minutes to progress UV polymerization, thus preparing ahydrogel polymer sheet.

The polymerized sheet was taken out and cut to a size of 3 cm×3 cm, andthen, chopped using a meat chopper to prepare crumb. The crumb was driedin an oven capable of transferring air volume up and down. Hot air of185° C. was flowed from the lower part to the upper part for 15 minutes,and flowed from the upper part to the lower part for 15 minutes, so thatthe crumb was uniformly dried, and the moisture content of the driedproduct became 2% or less. After drying, it was ground with a grinder,and then, sieved for 10 minutes with an amplitude of 1.5 mm (combinationof sieving mesh: #20-30/#30-50/#50-100), and each sieved part(5%/75%/20%) was collected to obtain polymer having a particle diameterof about 150 μm to 850 μm, thus obtaining base resin powder.

Thereafter, 0.025 parts by weight of glyceryl stearate, and a surfacecrosslinking solution (7.7 parts by weight of water, 5.5 parts by weightof methanol, 0.02 parts by weight of ethyleneglycol diglycidyl ether(EX-810), 0.05 parts by weight of sodium metabisulfite, 0.1 parts byweight of aluminum sulfate 18 hydrate (Al—S), and 0.03 parts by weightof aluminum oxide (Alu 130)) were uniformly mixed with 100 parts byweight of the base resin prepared above, and then, a surfacecrosslinking reaction was progressed at 140° C. for 35 minutes. Afterthe surface treatment was finished, superabsorbent polymer having anaverage particle diameter of 150 to 850 μm was obtained using a sieve.

Thereafter, 0.05 parts by weight of aluminum oxide (Alu 130) was drymixed with the superabsorbent polymer prepared above.

Example 5

100 g of acrylic acid, 0.75 g of polyethyleneglycol diacrylate (PEGDA,Mw=523) as a crosslinking agent, 0.008 g ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide as a photoinitiator,0.08 g of sodium persulfate (SPS) as a thermal initiator, 0.2 g ofsodium bicarbonate (SBC) as a foaming agent, 0.025 g of sodiumdodecylsulfate (SDS) as surfactant, 0.1 g of colloidal silica (ST-O),123.3 g of 31.5% caustic soda (NaOH), and 38.53 g of water were mixed toprepare an aqueous monomer composition. The aqueous monomer compositionwas received in a tray, and then, while maintaining a polymerizationatmosphere temperature of 80° C., it was irradiated by UV (irradiationamount: 10 mW/cm²) with a UV irradiation equipment for 1 minute, andaged for 2 minutes to progress UV polymerization, thus preparing ahydrogel polymer sheet.

The polymerized sheet was taken out and cut to a size of 3 cm×3 cm, andthen, chopped using a meat chopper to prepare crumb. The crumb was driedin an oven capable of transferring air volume up and down. Hot air of185° C. was flowed from the lower part to the upper part for 15 minutes,and flowed from the upper part to the lower part for 15 minutes, so thatthe crumb was uniformly dried, and the moisture content of the driedproduct became 2% or less. After drying, it was ground with a grinder,and then, sieved for 10 minutes with an amplitude of 1.5 mm (combinationof sieving mesh: #20-30/#30-50/#50-100), and each sieved part(5%/75%/20%) was collected to obtain polymer having a particle diameterof about 150 μm to 850 μm, thus obtaining base resin powder.

Thereafter, 0.2 parts by weight of glyceryl stearate, and a surfacecrosslinking solution (7.7 parts by weight of water, 5.5 parts by weightof methanol, 0.02 parts by weight of ethyleneglycol diglycidyl ether(EX-810), 0.05 parts by weight of sodium metabisulfite, 0.1 parts byweight of aluminum sulfate 18 hydrate (Al—S), and 0.03 parts by weightof aluminum oxide (Alu 130)) were uniformly mixed with 100 parts byweight of the base resin prepared above, and then, a surfacecrosslinking reaction was progressed at 140° C. for 35 minutes. Afterthe surface treatment was finished, superabsorbent polymer having anaverage particle diameter of 150 to 850 μm was obtained using a sieve.

Thereafter, 0.05 parts by weight of aluminum oxide (Alu 130) was drymixed with the superabsorbent polymer prepared above.

Comparative Example 1

100 g of acrylic acid, 0.6 g of polyethyleneglycol diacrylate (PEGDA,Mw=523) as a crosslinking agent, 0.008 g ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide as a photoinitiator,0.15 g of sodium persulfate (SPS) as a thermal initiator, 0.08 g ofsodium bicarbonate (SBC) as a foaming agent, 0.035 g of sodiumdodecylsulfate (SDS) as surfactant, 123.3 g of 31.5% caustic soda(NaOH), and 38.53 g of water were mixed to prepare an aqueous monomercomposition. The aqueous monomer composition was received in a tray, andthen, while maintaining a polymerization atmosphere temperature of 80°C., it was irradiated by UV (irradiation amount: 10 mW/cm²) with a UVirradiation equipment for 1 minute, and aged for 2 minutes to progressUV polymerization, thus preparing a hydrogel polymer sheet.

The polymerized sheet was taken out and cut to a size of 3 cm×3 cm, andthen, chopped using a meat chopper to prepare crumb. The crumb was driedin an oven capable of transferring air volume up and down. Hot air of185° C. was flowed from the lower part to the upper part for 15 minutes,and flowed from the upper part to the lower part for 15 minutes, so thatthe crumb was uniformly dried, and the moisture content of the driedproduct became 2% or less. After drying, it was ground with a grinder,and then, sieved for 10 minutes with an amplitude of 1.5 mm (combinationof sieving mesh: #20-30/#30-50/#50-100), and each sieved part(10%/75%/15%) was collected to obtain polymer having a particle diameterof about 150 μm to 850 μm, thus obtaining base resin powder.

Thereafter, a surface crosslinking solution (7.6 parts by weight ofwater, 7.6 parts by weight of methanol, 0.075 parts by weight ofethyleneglycol diglycidyl ether (EX-810), 0.03 parts by weight of sodiummetabisulfite, 0.1 parts by weight of aluminum sulfate 18 hydrate(Al—S), and 0.03 parts by weight of aluminum oxide (Alu 130)) wasuniformly mixed with 100 parts by weight of the base resin preparedabove, and then, a surface crosslinking reaction was progressed at 140°C. for 35 minutes. After the surface treatment was finished,superabsorbent polymer having an average particle diameter of 150 to 850μm was obtained using a sieve.

Thereafter, 0.05 parts by weight of aluminum oxide (Alu 130) was drymixed with the superabsorbent polymer prepared above.

Comparative Example 2

100 g of acrylic acid, 0.6 g of polyethyleneglycol diacrylate (PEGDA,Mw=523) as a crosslinking agent, 0.008 g ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide as a photoinitiator,0.15 g of sodium persulfate (SPS) as a thermal initiator, 0.08 g ofsodium bicarbonate (SBC) as a foaming agent, 0.035 g of sodiumdodecylsulfate (SDS) as surfactant, 0.4 g of silica (Aerosil 200), 123.3g of 31.5% caustic soda (NaOH), and 38.53 g of water were mixed toprepare an aqueous monomer composition. The aqueous monomer compositionwas received in a tray, and then, while maintaining a polymerizationatmosphere temperature of 80° C., it was irradiated by UV (irradiationamount: 10 mW/cm²) with a UV irradiation equipment for 1 minute, andaged for 2 minutes to progress UV polymerization, thus preparing ahydrogel polymer sheet.

The polymerized sheet was taken out and cut to a size of 3 cm×3 cm, andthen, chopped using a meat chopper to prepare crumb. The crumb was driedin an oven capable of transferring air volume up and down. Hot air of185° C. was flowed from the lower part to the upper part for 15 minutes,and flowed from the upper part to the lower part for 15 minutes, so thatthe crumb was uniformly dried, and the moisture content of the driedproduct became 2% or less. After drying, it was ground with a grinder,and then, sieved for 10 minutes with an amplitude of 1.5 mm (combinationof sieving mesh: #20-30/#30-50/#50-100), and each sieved part(10%/75%/15%) was collected to obtain polymer having a particle diameterof about 150 μm to 850 μm, thus obtaining base resin powder.

Thereafter, a surface crosslinking solution (7.6 parts by weight ofwater, 7.6 parts by weight of methanol, 0.075 parts by weight ofethyleneglycol diglycidyl ether (EX-810), 0.03 parts by weight of sodiummetabisulfite, 0.1 parts by weight of aluminum sulfate 18 hydrate(Al—S), and 0.03 parts by weight of aluminum oxide (Alu 130)) wasuniformly mixed with 100 parts by weight of the base resin preparedabove, and then, a surface crosslinking reaction was progressed at 140°C. for 35 minutes. After the surface treatment was finished,superabsorbent polymer having an average particle diameter of 150 to 850μm was obtained using a sieve.

Thereafter, 0.05 parts by weight of aluminum oxide (Alu 130) was drymixed with the superabsorbent polymer prepared above.

Experimental Example

For the superabsorbent polymer prepared in Examples and ComparativeExamples, the properties were evaluated as follows.

Unless otherwise indicated, all the property evaluations were progressedat constant temperature constant humidity (23±1° C., relative humidity50±10%), and a saline solution or brine means an aqueous solution of 0.9wt % sodium chloride (NaCl).

And, in the following rewet property evaluation, as the tap water, tapwater having electric conductivity of 170 to 180 μS/cm, when measuredusing Orion Star A222 (Thermo Scientific), was used.

(1) Centrifuge Retention Capacity (CRC)

Centrifuge retention capacity by absorption rate under no load wasmeasured according to EDANA WSP 241.3.

Specifically, W₀(g) (about 0.2 g) of superabsorbent polymer wasuniformly put in an envelope made of non-woven fabric and sealed, andthen, soaked in a saline solution (0.9 wt %) at room temperature. After30 minutes, it was drained for 3 minutes under 250G using a centrifuge,and the weight W₂(g) of the envelope was measured. And, the sameoperation was conducted without using polymer, and then, the weightW₁(g) at that time was measured. Using each obtained weight, CRC (g/g)was calculated according to the following Mathematical Formula.CRC (g/g)={[W ₂ (g)−W ₁ (g)]/W ₀ (g)}−1  [Mathematical Formula 1]

(2) Absorption Under Pressure (AUP)

Absorption under 0.7 psi pressure of each polymer was measured accordingto EDANA method WSP 242.3.

Specifically, on the bottom of a plastic cylinder having an innerdiameter of 60 mm, a 400 mesh wire netting made of stainless wasinstalled. Under room temperature and 50% humidity conditions, W₀(g)(0.9 g) of superabsorbent polymer was uniformly sprayed on the wirenetting, and a piston having an outer diameter slightly smaller than 60mm and capable of further giving 0.7 psi load was installed thereon sothat there was no gap with the inner wall of the cylinder and the up anddown movement was not hindered. At this time, the weight W₃(g) of thedevice was measured.

Inside a petri dish having a diameter of 150 mm, a glass filter having adiameter of 90 mm and a thickness of 5 mm was laid, and a salinesolution consisting of 0.9 wt % sodium chloride was put to the samelevel with the upper side of the glass filter. One piece of a filterpaper having a diameter of 90 mm was laid thereon. On the filter paper,the measuring device was laid, and the liquid was absorbed underpressure for 1 hour. After 1 hour, the measuring device was lifted, andthe weight W₄(g) was measured.

Using each obtained weight, absorption under pressure (g/g) wascalculated according to the following Mathematical Formula.AUP (g/g)=[W ₄ (g)−W ₃ (g)]/W ₀ (g)  [Mathematical Formula 2]

(3) Permeability

Permeability was measured according to the method described in U.S. Pat.No. 9,656,242 B2.

A device for measuring permeability was a chromatography column that hasan inner diameter of 20 mm, and is equipped with a glass filter on thebottom. While a piston was put in the chromatography column, lines weremarked at the liquid levels corresponding to the liquid amounts 20 mland 40 ml. Thereafter, between the glass filter on the bottom of thechromatography column and a cock, water was inversely introduced so asnot to generate bubbles, and filled up to about 10 ml, followed bywashing with brine twice to three times, and filling with 0.9% brine upto 40 ml or more. A piston was put in the chromatography column, thelower valve was opened, and a time (B) taken until the liquid leveldecreased from the marked line of 40 ml to the marked line of 20 ml wasrecorded.

In the chromatography column, 10 ml of brine was left, 0.2±0.0005 g of asieved (30 #˜50 #) superabsorbent polymer sample was put, and brine wasadded to the volume of 50 ml, and then, it was allowed to stand for 30minutes. Thereafter, a piston with weight (0.3 psi=106.26 g) was put inthe chromatography column, and allowed to stand for 1 minute, and then,the lower valve of the chromatography column was opened, and a time (T1)taken until the liquid level decreased from the marked line of 40 ml tothe marked line of 20 ml was recorded, thus calculating a time of T1−B(unit: second).

(4) Vortex Time

A vortex time was measured in the unit of seconds according to themethod described in International Patent Publication No. 1987-003208.

Specifically, into 50 mL of a saline solution of 23° C. to 24° C., 2 gof superabsorbent polymer was introduced, and while stirring with amagnetic bar (diameter 8 mm, length 30 mm) at 600 rpm, a time takenuntil vortex disappeared was measured in the unit of seconds, thuscalculating the vortex time.

(5) Tap Water Long-Term Rewet Under No Pressure (3 Hrs)

{circle around (1)} In a cup (upper part diameter 7 cm, lower partdiameter 5 cm, height 8 cm, volume 192 ml), 1 g of superabsorbentpolymer having a particle size of #30-#50 was put, and while stirring at500 rpm, 100 g of tap water was poured to swell the superabsorbentpolymer.

{circle around (2)} CD When the superabsorbent polymer was filled withtap water, the surface of water became flat, and the tap water did notflow, stirring was stopped.

{circle around (3)} CD 10 minutes after pouring tap water, the cupcontaining swollen superabsorbent polymer was turned over on 5 pieces offilter papers (manufacturing company: whatman, catalog No. 1004-110,pore size 20-25 μm, diameter 11 cm).

{circle around (4)} 3 hours after pouring tap water, the cup and thesuperabsorbent polymer were removed, and the amount of tap water (unit:g) wetted on the filter papers was measured.

(6) Tap Water Short-Term Rewet Under No Pressure (1 hr)

{circle around (1)} In a cup (upper part diameter 7 cm, lower partdiameter 5 cm, height 8 cm, volume 192 ml), 1 g of superabsorbentpolymer was put, and 100 g of tap water was poured to swell thesuperabsorbent polymer.

{circle around (2)} 10 minutes after pouring tap water, the cupcontaining swollen superabsorbent polymer was turned over on 5 pieces offilter papers (manufacturing company: whatman, catalog No. 1004-110,pore size 20-25 μm, diameter 11 cm).

{circle around (3)} 1 hour after pouring tap water, the cup and thesuperabsorbent polymer were removed, and the amount of tap water (unit:g) wetted on the filter papers was measured.

(7) Tap Water Long Term Rewet Under Pressure (6 Hrs)

{circle around (1)} In a petri dish having a diameter of 13 cm, 4 g ofsuperabsorbent polymer was uniformly sprayed and uniformly distributedusing a spatula, and 200 g of tap water was poured to swell thesuperabsorbent polymer.

{circle around (2)} The superabsorbent polymer swollen for 6 hours waspressurized for 1 minute with a weight of 5 kg having a diameter of 11cm (0.75 psi) on 20 pieces of filter papers having a diameter of 11 cm(manufacturing company: whatman, catalog No. 1004-110, pore size 20-25μm, diameter 11 cm).

{circle around (3)} After pressurizing for 1 minute, the amount of tapwater (unit: g) wetted on the filter papers was measured.

The property values of Examples and Comparative Examples were describedin the following Table 1.

TABLE 1 Compar- Compar- Example Example Example Example Example ativeative 1 2 3 4 5 Example 1 Example 2 CRC (g/g) 32.7 30.8 32.7 30.6 32.333.2 33.2 Vortex time 34 29 28 25 25 35 38 (seconds) 0.7 psi AUP 23.923.2 23 22 21 19.1 20.9 (g/g) Permeability 14 14 15 15 13 24 13(seconds) Tap water 2.8 2.8 2 2 1.7 5.5 3.5 long term rewet under nopressure (g) Tap water 0.7 0.75 0.6 0.65 0.5 1.5 1.3 short term rewetunder no pressure (g) Tap water 0.7 0.8 0.4 0.5 0.4 2 1.5 long termrewet under pressure (g)

Referring to Table 1, it was confirmed that in Examples 1 to 5 of thepresent invention, excellent vortex time and permeability wereexhibited, and tap water rewet amounts under no pressure and underpressure were very small, thus exhibiting improved rewet property.

To the contrary, it can be seen that in Comparative Examples 1 and 2,rewet and vortex time were inferior to Examples.

What is claimed is:
 1. A method for preparing superabsorbent polymercomprising: preparing a base resin in which acrylic acid-based monomershaving acid groups, of which at least a part are neutralized, and aninternal crosslinking agent are crosslinked, in the presence ofcolloidal silica; mixing a hydrophobic material having a HLB of 0 ormore and 6 or less, and a surface crosslinking agent with the base resinto obtain a surface crosslinking solution; and increasing a temperatureof the surface crosslinking solution to conduct surface crosslinking ofthe base resin.
 2. The method for preparing superabsorbent polymeraccording to claim 1, wherein a particle diameter of the colloidalsilica is 10 to 100 nm.
 3. The method for preparing superabsorbentpolymer according to claim 1, wherein the colloidal silica is includedin a content of 0.01 to 1.0 parts by weight, based on 100 parts byweight of the acrylic acid-based monomers.
 4. The method for preparingsuperabsorbent polymer according to claim 1, wherein the hydrophobicmaterial has a melting point below the increased temperature.
 5. Themethod for preparing superabsorbent polymer according to claim 1,wherein the hydrophobic material includes one or more of glycerylstearate, glycol stearate, magnesium stearate, glyceryl laurate,sorbitan stearate, sorbitan trioleate, or PEG-4 dilaurate.
 6. The methodfor preparing superabsorbent polymer according to claim 5, wherein thehydrophobic material is glyceryl stearate.
 7. The method for preparingsuperabsorbent polymer according to claim 1, wherein the hydrophobicmaterial is mixed in a content of 0.02 to 0.5 parts by weight, based on100 parts by weight of the base resin.
 8. The method for preparingsuperabsorbent polymer according to claim 1, wherein during the mixing,the hydrophobic material is dry mixed with the base resin, and then, asurface crosslinking solution comprising the surface crosslinking agentis mixed therewith.
 9. The method for preparing superabsorbent polymeraccording to claim 1, wherein the temperature is increased to 80 to 190°C.
 10. The method for preparing superabsorbent polymer according toclaim 1, wherein the preparing the base resin comprises: polymerizing amonomer composition comprising acrylic acid-based monomers having acidgroups, of which at least a part are neutralized, an internalcrosslinking agent, colloidal silica, and a polymerization initiator toform a hydrogel polymer; drying the hydrogel polymer; grinding the driedpolymer; and sieving the ground polymer.
 11. The method for preparingsuperabsorbent polymer according to claim 10, wherein the monomercomposition further comprises a foaming agent and a foam stabilizer. 12.The method for preparing superabsorbent polymer according to claim 11,wherein the foaming agent includes one or more of sodium bicarbonate,sodium carbonate, potassium bicarbonate, potassium carbonate, calciumbicarbonate, calcium bicarbonate, magnesium bicarbonate or magnesiumcarbonate.
 13. The method for preparing superabsorbent polymer accordingto claim 11, wherein the foam stabilizer includes one or more of sodiumdodecyl sulfate, sodium stearate, ammonium lauryl sulfate, sodium laurylether sulfate(SLES), or sodium myreth sulfate.
 14. The method forpreparing superabsorbent polymer according to claim 1, wherein thesuperabsorbent polymer has a vortex time of 35 seconds or less.
 15. Themethod for preparing superabsorbent polymer according to claim 1,wherein the superabsorbent polymer has a permeability (unit: seconds)measured according to the following Equation 1, of 20 seconds or less:Permeability (sec)=T1−B  [Equation 1] in the Equation 1, T1 is a timetaken until the height of a liquid level decreases from 40 ml to 20 ml,after putting 0.2±0.0005 g of a sieved (30#˜50#) superabsorbent polymersample in a chromatography column and adding brine to the volume of 50ml, and then, leaving it for 30 minutes; and B is a time taken until theheight of a liquid level decreases from 40 ml to 20 ml in achromatography column filled with brine.